There is a known issue that increases the damage of each flechette when using the Heavy Barrel. Instead of doing little damage, each Flechette gains the damage of a bullet fired from the primary weapon, i.e. 34 damage per flechette if using the G3A3. I believe there is also an increase in range. You may not want to try it too much though, as many people will quickly hate you for it, screaming OP.
While I will agree that the glitch is an issue and needs to be fixed, there needs to be something done with the pump action shotguns. Using the 870 MCS with Buckshot/Flechette to unlock Slugs, one shot kills at the appropriate distances were wildly inconsistent. There were many times I thought I would get a one shot kill, only to receive a hit marker or no hit marker at all. Then there were times when I was certain I would need more than one shell to achieve a kill and one shell ended up being enough. When using a weapon that is designed for extreme close quarters and using the proper shells for extreme close quarters, hit markers are unacceptable, as there is very little margin for error, especially when encountering more than one enemy. I died quite a few times in multiple enemy encounters because of hit markers in very close ranges.
Suppression was also incredibly devastating to the 870 MCS when using Buckshot/Flechette. If I was not the first to get a shot off against a fully automatic weapon, then I was likely going to die, as the suppression effects stick around much too long and cause a ridiculous spread of Buckshot/Flechette. In turn, I was much better off switching to my sidearm, as the spread for a pistol while being suppressed is not nearly as bad as what happens to Buckshot/Flechette. Seems very counter intuitive to switch to another weapon in close quarters combat when I am already using a weapon designed for such encounters.
Now that I have unlocked Slugs for the 870 MCS, I will not go back to Buckshot or Flechette shells, unless something changes. Slugs are nearly as effective at one shot kills as with Buckshot or Flechette in close quarters. Assuming you hit the person in the chest or head at close range, then you are going to achieve one shot kills in very close range, much like you would with Buckshot or Flechette. Unlike Buckshot/Flechette though, Slugs can be effectively used at pretty long ranges--I am not talking sniper rifle distance, but to a point where you are not completely gimp against an assault rifle, carbine, or LMG and can somewhat reasonably compete by landing headshots--and make a limited weapon, the shotgun, into a more versatile weapon.